The present invention relates to electric motors and, more particularly, to commutator/brush electric motors of the type used in photographic camera applications.
Miniature electric motors have found wide-spread application in many consumer products including self-developing photographic cameras (such as the PRONTO! camera) that are loaded with a plurality of self-developing film units contained within a film pack. In these types of cameras, a miniature, battery operated electric motor is employed to drive the film eject mechanism for a carefully determined period, usually 1.5 seconds, to eject each film unit after exposure. The motor typically drives a multiple pass, step-down gear train that is connected to and rotates pressure applying rollers so that the exposed film unit passes between and through the rollers as it is ejected from the camera. Motor efficiency is vital to reduce battery drain, while reliability and long life are also necessary. Further, it is an operational requirement of the drive motor that the film unit be ejected within a narrowly specified rate to permit uniform development. Hence, out of specification drive motor performance, including out of specification start-up torque and run speed, can impair the quality of the developed film unit.
Electric motors typically used in the above-described application include a permanent magnet stator and a rotor assembly supported for rotation by bearings mounted in the motor end caps. The rotor assembly includes an armature stack and a plurality of armature coils wound on the armature stack. A multi-bar commutator is mounted on the rotor shaft adjacent one end of the armature stack with the individual commutator bars electrically connected to the armature coils to define a complete armature circuit.
A brush assembly is usually mounted in the stator with brushes resiliently contacting the commutator to effect commutation. Each brush typically is a multi-element structure including a resilient cantilevered beam fabricated from, for example, beryllium copper, phosphur bronze, and/or nickel copper alloys carrying a brush contact secured to one end so as to be resiliently biased against the commutator. Typically, the brush contact contains precious metal such as silver, for example silver graphite or the like, designed to effect optimum electrical contact with the commutator bars during commutation.
The rotor shaft is usually mounted in sleeve bearings formed of sintered bronze impregnated with bearing oil. Typical bearing oils, while providing extended bearing life, can cause commutation problems if they migrate to the commutator/brush interface since they would deposit an oil film at the brush/commutator interface which is transformed into a solid carbonaceous deposit that is a high impedance film which degrades commutation when the oil is exposed to the electric field between the commutator and the brush contact and the make-break sparking or brush fire that occurs during commutation. Also, any bearing oil absorbed into the brush contact structure can also adversely affect performance. An electric motor having its commutator and brushes contaminated in this manner can exhibit uneven start-up torque and run speed characteristics. In order to minimize the migration of the bearing oil from the shaft bearing to the commutator, it has been a standard practice in the industry to position a washer on the rotor shaft between the commutator and the support bearing to serve as an oil slinger, when the motor is in operation, to throw off any oil that may have migrated to the slinger from the bearing. The slinger functions as a dynamic barrier in that it is operational only when the rotor is rotating.
The aforesaid motor design has proven satisfactory as the drive motor in the film unit ejection mechanism of self-developing cameras although its design does present some problems since, like all cameras, operating conditions can vary from near continuous use on a daily basis to a single exposure use between substantial periods of non-use. In substantial periods of non-use, often lasting several months or more, bearing oil can migrate from the bearing/shaft interface. As a result, the bearing interface from which the oil migrated can be left with insufficient surface oil for optimum lubrication at start-up, and the aforedescribed oil on the commutator and oil in the brush contacts condition can be present. During the next period of camera operation, the motor performance, including start-up torque and run speed, can deteriorate and, as can be appreciated, degraded motor performance can result in uneven passage of the exposed film unit through the pressure applying rollers and less than optimum image development.
As can be also appreciated from the foregoing, there is a substantial need in camera applications for a miniature, economical electric motor of high efficiency to reduce battery drain, and high reliability to exhibit long life and in-specification, first-time performance after substantial periods of non-use.